interbasin fluvial system is recorded in the Cretaceous Alter do Chão Formation [3], but with increased Neogene Andean uplift and tectonic subsidence in western Amazonia this regime gave way to isolated sedimentary basins east and west of the Purus

INTRODUCTION Geological evolution of the Amazon Basin. The geological history of lowland Amazonia is still poorly understood, in part because much of the Tertiary stratigraphic history has been removed by erosion and that which remains is difficult to study and access in a rainforest environment. The landscape evolution of Amazonia during the Neogene was influenced mainly by Andean tectonics and climate changes that controlled the distribution of sedimentary environments. The most important natural barrier has been the Purus Arch, a NW-SE trending tectonic structure located 300 km to the west of Manaus that separates the Solimões and Amazonas basins [1]. This feature was a Neoproterozoic basin filled by sandstones, mudstones and carbonates and tectonically inverted during Early Paleozoic times. Evidence of inversion includes inclination of Purus Group beds up to 10 degrees towards the south, as well as the onlap and pinch-out disposition of Paleozoic units on the Purus Arch. Additionally, the thickness of Phanerozoic rocks is dramatically reduced in the region of the Arch suggesting an uplifted block. This geometry indicates that the Purus Arch was a compartmentalized zone during the Phanerozoic evolution of the Solimões and Amazon basins, controlling, in part, the lateral migration of the depositional system [1,2]. A westward-flowing interbasin fluvial system is recorded in the Cretaceous Alter do Chão Formation [3], but with increased Neogene Andean uplift and tectonic subsidence in western Amazonia this regime gave way to isolated sedimentary basins east and west of the Purus Arch [1]. The long-accepted model postulating the formation of the Amazon River drainage in the Middle to Late Miocene relies relatively little on the Late Miocene (post-Pebasian) and Plio-Pleistocene stratigraphic record in the Solimões Basin [e.g., 3-7], hence it was the loss of the Miocene Lake Pebas and its aquatic environments that has been used as the signature evidence for the initiation of terrestrial environments and the establishment of the modern drainage system in western Amazonia (but see below). In contrast, the hypothesis that the Amazon River and its western drainage were established in Plio-Pleistocene times [2, 8, 9] relies heavily on evidence presented by stratigraphic sections of Late Miocene to Recent age. The Lake Pebas aquatic environments of the Early and Middle Miocene of western Amazonia are recorded in the Solimões Formation (which extends eastward to the Purus Arch; 2) and the "Pebas Beds." These are overlain by weathered paleosols and then by an unconformity (termed the …